Foamed thermoplastic polymers enjoy widespread use in building construction due to their low density and excellent insulating properties. For these uses, flame-resistant, self-extinguishing foams are desirable. Many halogenated organic compounds have been proposed as additives for improving flame resistance. Some of these flame retardants would be effective and economical, but are not widely used because they inhibit monomer polymerization or impart severe heat sensitivity to the pre-expanded products.
The manufacture of molded articles from expanded thermoplastic polymer particles such as expandable polystyrene beads (EPS) is well known. Two methods are commonly used for preparing molded products from vinyl aromatic monomers. In one method, called herein "the two-step process," the vinyl aromatic monomer, typically styrene, is suspension polymerized to form hard polymer beads, which are isolated and screened to the desired size distribution. The sieved beads are then resuspended in water, impregnated with a blowing agent--optionally in the presence of a flame retardant, pre-expanded with steam, aged, and molded. This process is illustrated by the teachings of U.S. Pat. Nos. 4,028,285 and 4,113,672.
In another method, called herein the "one-step process," the vinyl aromatic polymer is suspension polymerized--often in the presence of a flame retardant--and the beads are impregnated with a blowing agent and cured without isolation or screening of the beads as in the two-step process. The one-step process for making expandable beads is illustrated by the teachings of U.S. Pat. Nos. 4,286,071, 3,975,327, 3,755,209, and 4,281,067. While the one-step method is simpler, the beads produced tend to have a wider size distribution, which limits the number of suitable applications for beads made in this way.
Flame retardants are typically incorporated into vinyl aromatic polymer beads either during polymerization or impregnation. These methods are summarized by P. L. Ku in "Fire Retardants for Readily Combustible Cellular Polystyrene," Adv. Polm. Tech. 9 (1989) on pages 63, 66 and 67. 1,2,5,6,9,10-hexabromocyclododecane (HBCD) is by far the most commonly used flame retardant for either process. In the one-step process, the flame retardant is included at the start of the polymerization. In the two step process, the flame retardant is added either initially, or during impregnation of the hard beads with blowing agent.
A disadvantage of adding the flame retardant before polymerization in the one-step process is that the flame retardant must be compatible with the suspension polymerization mixture. If the flame retardant inhibits polymerization of the vinyl aromatic monomer or renders the expandable beads heat sensitive, molding of satisfactory products may not be possible. This limits the number of flame retardants useful in the one-step process. Another disadvantage of the conventional method for adding flame retardant early in the one-step process is the need to include additional reagents in the polymerization mixture such as peroxide synergists or acid scavengers. New methods of adding flame retardant in the one-step process are needed as alternatives to the conventional approach of adding flame retardant into the polymerization mixture. Methods are needed that allow one to add otherwise desirable flame retardants that are not currently useable because of their tendency to inhibit vinyl aromatic monomer polymerization. Methods that reduce the number of required components in the polymerization mixture and thus allow simpler, less expensive formulations are also desirable.